In recent years, femtosecond light pulses have been utilized in a variety of forms for controlling molecular states, the electronic state of a solid, and chemical reactions and machining a material (see, for example, References 1 to 5 listed below). Also, as the application of femtosecond light pulses is expanding, demands have increased for a pulse generator for a femtosecond light pulse that is shorter in time width and for a femtosecond light pulse applied apparatus that excels in cost reduction and easiness for using.
Of importance to meet with these demands is the technique of linearly chirping the frequency of femtosecond light pulses. Frequency chirping is the phenomenon that the instantaneous frequency of a light pulse changes with time, wherein the case that it increases linearly with time is called positive chirping and the case that it decreases linearly with time is called negative chirping.
Generating a femtosecond light pulse is by way of a technique whereby relative phases between spectral components of a light pulse spreading in time width are controlled to compress the time width to a limit of the Fourier transformation. Since a light pulse is spread on the time axis according to a relationship in relative phases between spectral components of the light pulse, the light pulse is compressed by compensating for relative phases with respect to frequency, namely by chirping (see, for example, Reference 6). Linear chirp as it occurs when the propagation constant of light has constant group velocity dispersion, namely dependence on the square of frequency, is also referred to as secondary dispersion.
While demands to further narrow the time width of a femtosecond light pulse are rising, the fact that narrowing the time width of a light pulse expands its frequency spectral bandwidth correspondingly makes it necessary to impart a chirp to the ranges all between the expanded frequency spectral components and thus to impart a large chirp to frequency ranges wider than those heretofore.
Further, with the recent discovery that the electronic state of a material brought about when it is irradiated with femtosecond light pulses is varied according to the direction of a chirp, it has become necessary to precisely control the direction and amount of a chirp in order to synthesize a new material on the basis of such a new principle. Also, in the field of optical communication, in order to remove or reduce the time spread of a light pulse signal, or the time delay between successive wavelength signals in WDM (wavelength Division Multiplexing), there has become necessary a chirp control apparatus which can control or change the direction of a chirp and the magnitude of its amount as desired and which excels in cost reduction and easiness for using. While demands for the linear chirp technique have thus been growing considerably, they can hardly be met by conventional linear chip techniques as described below.
While it is generally easy to impart a positive chirp to a light pulse, imparting a negative chirp thereto has called for a complicated mechanism. A typical apparatus in the prior art for controlling a chirp amount employs a prism pair or a pair of diffraction gratings wherein the distance between such optical elements in the pair is varied to change the amount of a negative chirp (see Reference 7). However, the attempt to apply a large negative chirp over an extended frequency spectral range gives rise to the problem that there come to be imparted not only the linear chirp, namely secondary dispersion but also dispersions higher in order than the secondary.
There has also been a chirp control apparatus using liquid crystal devices or variable diffraction grating mirrors which to spatially disperse the spectrum of a light pulse with dependence on its frequencies are spatially disposed at different positions for different frequencies whereby the frequency components after each of them has a given phase imparted thereto are added together. Such an apparatus has the problem, however, that among others, the liquid crystal device is low in damaging threshold to light energy and thus can not endure the use of a high-energy light pulse and also the problem that the apparatus itself must be large in size, thus becoming costly and poor in easiness for using.
There has further been a chirp control apparatus using a dielectric multilayer film mirror (see Reference 8) which is formed from two or more optical films different in index of refraction (dielectric constant) and by laminating such films alternately a plurality of times while controlling their thicknesses so that light reflected from the laminate has a phase proportional to frequency. This apparatus using optical materials, such as SiO2 and TiO2, which are high in the damaging threshold to light energy, to form the dielectric multilayer film can well endure the use of high-energy light pulses. Further, the apparatus simply having a dielectric multilayer film mirror reflect a light pulse is small in size.